1. Field of the Invention
The present invention is directed to a method for controlling the operation of a roller grinding mill having its output connected to a burner such that a steam flow requirement can be produced at a substantially constant level.
2. Description of the Prior Art
In the past fuel feeders connected to roller mills have been traditionally controlled by measuring the relative pressure differential across the roller mill and using the differential as a target control means for the feeder to the roller mill. It has been the normal way of operating a roller mill in order to control the quantity of fuel in the grinding chamber so the rollers are able to perform efficiently in producing a satisfactory output.
There has been disclosed in the prior Williams U.S. Pat. No. 4,184,640 of Jan. 22, 1980 coal grinding apparatus employing a roller grinding mill for grinding coal to be supplied as the fuel for firing burners. In that arrangement the roller mill and a rotary classifier were connected in series and speed regulated so the mill and classifier could be modulated on an inverse speed relationship to maintain a uniform size of coal particles.
While the apparatus of the foregoing patent operates in a satisfactory manner, when it is desired to develop a substantially constant or nonfluctuating steam flow the technique of maintaining a differential pressure across the mill fails to deliver an output that is coordinated with a uniform steam flow output by the burner. As a result, the steam flow fluctuates in an undependable manner.
A further example of the prior art is found in Dickey U.S. Pat. No. 2,172,317 of Sept. 5, 1939 which has disclosed as bowl mill feed control for quickly varying the mill output in accordance with changes in the vapor pressure generated by the furnace, which vapor pressure affects the increase or decrease in the quantity of carrier air flow from the mill, and that change causes a change in the differential pressure across the orifice in the outlet duct from the classifier for the product reduced in the mill. However, this reference does not change mill speed as the grinding is determined by the spring setting on the rollers, and not by centrifugal response of the grinding rollers to speed.
The problem with these earlier arrangements is that they are based on a technique of relying on a differentl pressure associated with the output of a roller mill or pulverizer which fails to allow a direct fired burner to deliver an even or steady steam flow. A fluctuating steam flow from a burner above or below the demand is undesirable and inefficient.